The invention concerns a method of forging precipitation hardening type stainless steel.
At forging processing of a steel, if deformation resistance of the steel is high and life of tool is short, the material is usually reheated to a suitable temperature to decrease the deformation resistance, and then subjected to forging.
In case of precipitation hardening type stainless steels such as those having martensitic structure, e.g., SUS630, as is well known, because the martensitic structure is very hard and the deformability thereof is low, it is necessary to heat the material to be processed to a high temperature such as 600xc2x0 C., so that deformation resistance of the material may be low (i.e., 70 kgf/mm2 or less) and that the tool lives may be long.
Generally, in forging processes, for the purpose of preventing seizure of the work to the tools such as dies it is often practiced to form lubricating film (e.g., graphite coating) on the work or the die, or to spray a cooling oil to coat the dies. However, at a high temperature as 600xc2x0 C. or higher either the lubricating agent or the cooling oil may deteriorate by being oxidized, and therefore, these countermeasures are not effective.
Thus, it has been the actual fact that the precipitation hardening type steels can only be processed by machining to form the desired shape or by forging at such a low temperature as about 300xc2x0 C. Forging at this low temperature may not give products of complex shapes or high forging ratios.
The object of this invention is to solve these problems by providing an improved method of forging precipitation hardening type stainless steels. The method of forging of this invention comprises: soaking a precipitation hardening type stainless steel at a temperature of austenitizing range; cooling the steel to a temperature of 200-700xc2x0 C.; and forging the steel at this temperature.
After being heated to a temperature of austenitizing range and subsequently cooled to a temperature of 200-700xc2x0 C., preferably 400-600xc2x0 C., the structure of the material, which was once austenitized, remains as it is and is not transformed to martensite structure. Thus, the invention makes it possible to process the material in the condition of high ductility and softness of austenite structure. Forging may thus be possible to carry out at such a low temperature as less than 700xc2x0 C., approximately 200xc2x0 C. Therefore, the conventional lubricating film forming agents and die cooling oil can be used, and processing of higher forging ratios, which as been considered quite difficult, can be carried out.